Let me see if I know understand it.
You have two computers with compatible Windows systems.
You want to have both computers to have you current project stuff.
You have a 8GB USB Flash drive.
Now you got an error using drowp and drag.
Is so, then here is want comes up in my bird brain.
Maybe you have one or more of these:
1. A block of files are too large. Patio says the limit is 2GB
2. One of you drives, or the USB Flash, has a defect.
3. One of more of the files has been locked. By Who?
4. Somehow you crated a file that has a permission issue. It can not be copied to a USB device using FAT32.
Suggestion: Have you tried using the Briefcase? It was intended to do what you want. It copies over just whatever is new or modified.